Dr Haneen Khreis, who led the research at the University of Leeds, said: "Traffic-related air pollution is a real concern to the community.

"Quantifying the number of childhood asthma cases that are directly attributable to traffic-related air pollution has not been done in the past and as we show now, a significant portion of cases is largely preventable."

About 5.4m people in the UK are living with asthma, including 1.1 million children, according to the charity Asthma UK.

Professor John Wright, director of the Bradford Institute for Health Research, said: "This important study adds to the overwhelming evidence that air pollution is harming our children.

"The good news is that we can all save lives by driving less and using cleaner fuels."